1. Field of the Invention
The present invention relates to a high voltage generating apparatus and a method thereof. More specifically, the present invention relates to a high voltage generating apparatus implemented in an ASIC (application-specific integrated circuit) and a method thereof, in which the output voltage can be controlled using a digital control code.
2. Description of the Related Art
An image forming apparatus is an apparatus for printing on a recording medium, such as printing paper, an image that corresponds to inputted original image data. Examples of image forming apparatuses include printers, copy machines, laser printers, LED print head (LPH) printers and facsimile machines. In such image forming apparatuses, an electro-photographic method as been adopted. An electro-photographic image forming apparatus performs printing operations through the processes of charging, exposing, developing, transferring, fixing, and the like.
FIG. 1 is a cross-sectional view schematically showing a conventional electro-photographic image forming apparatus. Referring to FIG. 1, an electro-photographic image forming apparatus includes a photosensitive drum 10, charging roller 20, laser scanning unit (LSU) 30, developing roller 40, transferring roller 50, controller 60, and high voltage generating apparatus (HVPS) 70.
The operations of an electro-photographic image forming apparatus are now explained. First, under control of the controller 60, the high voltage generating apparatus 70 applies a predetermined voltage to the charging roller 20, developing roller 40, and transferring roller 50. The charging roller 20 evenly charges the surface of the photosensitive drum 10 with the charging voltage applied from the high voltage generating apparatus 70. Then, the LSU 30 scans light corresponds to the image data received from the controller 60 onto the photosensitive drum 10. Accordingly, an electrostatic latent image is formed on the surface of the photosensitive drum 10. Then, a toner image is formed on the electrostatic latent image by the toner supplied from the developing roller 40. The transferring roller 50 is driven by the transferring voltage applied from the high voltage generating apparatus 70. The transferring roller 50 transfers the toner image formed on the photosensitive drum 10 to the printing paper. Then, the toner image transferred on the printing paper is fixed on the printing paper by high temperature and pressure from a fixer (not shown). Finally, the printing paper is ejected in an ejecting direction (not shown), thereby completing a printing operation.
As described above, the high voltage generating apparatus 70 is a core element of a printer, copy machine, laser printer, LED print head (LPH) printer, facsimile machine and the like. The high voltage generating apparatus 70 is a device that instantly converts a low voltage of about 12˜24 V into a high voltage of hundreds˜thousands volts. The high voltage is applied to a roller of an electro-photographic image forming apparatus. The high voltage generating apparatus 70 is used as either a constant voltage or a constant current source.
FIG. 2 is a circuit diagram showing an example of a conventional high voltage generating apparatus. Referring to FIG. 2, a conventional high voltage generating apparatus comprises a low-pass filter 71, voltage controller 72, oscillator and power transformer 73, voltage distributor 74, voltage sensor 75, and a protection circuit 76. The low-pass filter 71 receives a pulse width modulation (PWM) signal D(t) from a print controller or the like. The low-pass filter 71 outputs a direct current (DC) voltage signal that is determined according to a duty ratio of PWM signal D(t). In doing so, the low-pass filter 71 converts the received PWM signal into a DC signal using a RC two-step filter. The outputted DC signal is used as a reference signal by voltage controller 72.
Voltage controller 72 operates as a differential circuit by comparing the DC signal outputted from the low-pass filter 71 and the feedback signal of high voltage generating apparatus's actual output voltage. Further, voltage controller 72 operates as a controller by generating a drive signal, based on the comparison, for controlling the amount of base current for transistor Q of the oscillator and power transformer 73. According to the voltage change between both ends of the transistor's emitter and collector, the voltage of the voltage transformer's primary coil is changed and likewise the voltage at the voltage transformer's secondary coil, having a higher turn ratio, is changed.
The voltage distributor 74 uses rectifying diodes D1 and D2, and voltage distributing and smoothing capacitors C4 and C5. Further, voltage distributor 74 generates the high voltage generating apparatus's DC output voltage from the alternating current (AC) voltage induced at the secondary coil of the power transformer. The voltage sensor 75 and the protection circuit 76 detect the actual output voltage and create a feedback signal for the voltage controller 72, and thus prevent an abnormal voltage from being applied.
FIG. 2 is the circuit diagram of a high voltage generating apparatus for generating a high voltage for one specific channel, such as the developing roller. A separate channel is respectively needed in order to apply similar high voltages to the charging roller 20, developing roller 40, and transferring roller 50.
The above exemplary conventional high voltage generating apparatus has a number of drawbacks. For instance, since it uses an analog control method to control the output of each channel independently, errors due to variances in the characteristics of the components that make up the RC filter or voltage controller must be compensated. In order to compensate, a plurality of elements are required thus making it is difficult to reduce the unit cost. A further drawback is that the poor quality of components, due to external factors, may cause the whole apparatus to malfunction. Moreover, another drawback is that the transistor used as a switching element in the oscillator and power transformer always operates in a linear area, thus the transistor always has an exothermic characteristic. Furthermore, as shown in FIG. 2, the conventional high voltage generating apparatus uses a plurality of elements which increases the number of man-hours required in the assembly process. Also, since there are many components, sufficient space needs to be allocated on the printed circuit board (PCB) for the numerous components. Lastly, since the elements are connected in a fixed manner, the output voltage cannot be easily controlled to vary.
Accordingly, there is a need for an improved high voltage generating apparatus that reduces the amount components required and is controllable such that its operation can be varied